Run xml linters on xsd and xslt files

Both xsd and xslt are by definition written in XML, and thus the same
linter(s) can be run to check them for well-formedness.

autoload/ale/linter.vim

@@ -17,6 +17,8 @@ let s:default_ale_linter_aliases = {
 \   'verilog_systemverilog': ['verilog_systemverilog', 'verilog'],
 \   'vimwiki': 'markdown',
 \   'vue': ['vue', 'javascript'],
+\   'xsd': ['xsd', 'xml'],
+\   'xslt': ['xslt', 'xml'],
 \   'zsh': 'sh',
 \}
 

doc/ale.txt

@@ -1016,6 +1016,8 @@ g:ale_linter_aliases                                     *g:ale_linter_aliases*
   \   'verilog_systemverilog': ['verilog_systemverilog', 'verilog'],
   \   'vimwiki': 'markdown',
   \   'vue': ['vue', 'javascript'],
+  \   'xsd': ['xsd', 'xml'],
+  \   'xslt': ['xslt', 'xml'],
   \   'zsh': 'sh',
   \}
 <